Method and apparatus for dewatering paper



Sept. 22, 1964 c. A. LEE

METHOD AND APPARATUS FOR DEWATERING PAPER Filed March 20, 1962 INVENTOR. 67/4/9156 A. 155

60AN5j A/VDEPSOM minim a F/rch United States Patent 3,150,036 METHOD AND APPARATUS FGR DEWATERHJG PAPER Charles Allen Lee, Knoxville, Tenn, assignor to Huyck Corporation, New York, N.Y., a corporation of New York Filed Mar. 20, 1962, Ser. No. 181,016 9 Claims. (Cl. l62-2tl5) The present invention relates generally to papermaking and, more particularly, it relates to improved methods and means for dewatering a sheet on a papermaking machine.

in the manufacture of paper and paperboard, a continuous sheet of cellulosic fiber is formed upon a supporting medium. This sheet initially comprises large quantities of water and has very little strength. Accordingly, it is necessary to provide a carrier for the sheet until sufiicient moisture is removed therefrom to provide a selfsupporting sheet.

In the case of a Fourdrinier papermaking machine, initial removal of water, or dewatering, is accomplished in the wire section of the machine by the use of gravity drainage from the wire which carries the formed paper web aided by table rolls and suction boxes. The sheet is thereafter removed from the wire at a couch roll, where it is conventionally transferred to an absorbent felt usual- 13/ of wool or the like. Adidtional dewatering is then effected by passing the web and its supporting felt through a series of press rolls called the press section, whereby water is expressed from the sheet, the water being absorbed by the felt or, if there is an excess of Water, running from the rolls in the manner of water from a clothes wringer.

Following the press section, the sheet is conducted over and between dryer rolls, which are heated and which effect removal of most of the remaining Water by evaporation, to provide a paper product of the desired moisture content. It is desirable to remove as much water as possible from the web on the Fourdrinier wire and in the press section of the machine in order to reduce the amount of heating which is required in the drying rolls to complete the moisture removal process.

In the press section several difiiculties appear unless care is taken. First, there is the problem of the interstices of the absorbent felt filling up with fines from the paper making fibers. This is remedied by cleaning the felt after each pass through the press section; however, even with the usual cleaning procedures the felt, when used with certain kinds of papers, becomes clogged with fines and must be often changed. This results in expense as a result of the expenditure of felt and the loss of production incident to the shut-down period required to change the felt. Second, there is the problem of crush which occurs when too much pressure is applied to the wet paper web at any given stage in the dewatering process. (Crush refers to a defect in paper, the formation of which has been broken by running it too wet through the press section and which manifests itself by mottling on the paper surface.) Because of the crushing problem, pressures must be carefully controlled on the various pairs of rolls in the press section and because of this problem, a substantial number of pairs of rolls are required, and the paper coming from the press section contains in the neighborhood of 65-70 percent moisture for most types of paper.

According to the present invention, a porous, permeable and substantially incompressible carrier of particular design is substituted for the conventional supporting felt in the press section. Thecarrier provides support for the paper web as it is brought to, through and awayfrom the press rolls. Being substantially incompressible and having a relatively low void volume, the carrier achieves improved dewatering without crushing, as will hereafter be explained.

It is an object of the present invention to provide an improved method and means for dewatering a cellulosic sheet. A more particular object of the present invention is to provide a method and means whereby increased amounts of moisture are removed from the sheet in the press section of a papermaking machine. Other objects and advantages of the present invention will become apparent from the following description and claims, taken in conjunction with the accompanying drawing in which:

The single figure shows one form of apparatus for removing water from a paper web.

I have discovered that the problem of crush may be minimized if not eliminated and that the efliciency of the press section may be greatly enhanced by modifying the press section in accordance with the present invention. In accordance with the present invention, the pressing, is accomplished while thewet web 10 is supported upon an endless porous and permeable carrier medium 12 of particular design in such a way that preferably substantially all of the water expressed from the web in the nip 14 of the press rolls 16, 18 is permanently removed from the webby passing through the web and the carrier in a direction generally perpendicular to the plane of the web and carrier.

Means is provided to deposit the wet web 10 on the carrier 12. As shown in the figure the web is transferred from a Fourdrinier wire 20- at a couch roll 22 and deposited on the carrier 12. The carrier is supported for endless movement on a number of rolls, such as roll 24. The carrier 12, with the web 10, is caused to move between the press rolls 16, 18 by driving the press rolls, as by gears 26. Force applied to bracket 28 applies the necessary force to press roll 16 to create the desired pressure. As shown, the carrier 12 is'supported so as to furnish support for the web 10 after leaving the press rolls 16, 18. As shown, press roll 18, which is the press roll in contact with the permeable carrier medium, is of open construction. Open construction includes a perforated roll, a slotted roll or any other rolls of similar character which will permit expressed water to run freely through the porous. carrying medium.

Ashas been pointed out above, the permeable carrier medium is of a particular construction. In the case of an ordinary papermakers felt porosity and permeability vary inversely with the amount of pressure that is exerted upon it. Thus, as a wet web on a felt enters the pressure nip of a pair of pressure rolls, the permeability of the felt becomes minimized at the point of maximum pressure and any water expressed from the wet web cannot readily flow through or into the felt. This causes the free water to flow back laterally in the web toward the inlet side of the nip and'thus causes derangement of the fibers and crushing. In accordance with the present invention, water flows in the web generally perpendicularly of its surface. into the carrier.

In accordance with the present invention, the wet web is supported upon an endless substantially incompressible. carrier fabric, preferably made from synthetic fibers, so that the permeability of the carrier does, not vary substantially with pressure. Since the carrier is dimensionally stable under pressure, its permeability will not change as the carrier and web passthrough the pressure nip; unlike the permeability of conventional felts. Further, the fabric desirably has particular dimensional characteristics. The permeabilityof the fabric should desirably be such that the amount of water in the wet web maypass into the fabric under the pressure in the, nip of a given pair of rolls in the time that itftak es the travelling web and fabric combination to pass through the rolls. This avoids any excess of water which might then be forced backwardly through the web and cause crushing. The carrier is permeable both perpendicularly to its surface and laterally. This permits water to flow generally perpendicularly to the surface from the paper web into the pores of the carrier and then laterally and generally perpendicularly through the carrier into the holes in the open roll, where the water is permanently extracted. This limits lateral movement of water in the paper web and the consequent crushing. It has been discovered that after the fabric and web assemblage passes from the nip, the paper web reabsorbs the water remaining in the fabric. Accordingly, the void volume of the fabric should be less than the amount of water which is to remain in the sheet when it is transferred from the fabric on which it is pressed. The void volume of a fabric is that part not filled with fiber; that is, it is the pore volume of the fabric available to be filled with water as the fabric and web are pressed together. (Void volume may be conveniently expressed in units of cubic inches of void per one hundred square feet of fabric. This may be determined by measuring the apparent volume geometrically and then subtracting the volume of the fibers in the fabric as determined by displacement.) The void volume also should preferably not vary substantially with pressure.

' More particularly in accordance with the present invention, the wet web during pressing is supported upon an endless substantially incompressible, permeable fabric which is preferably made from fibers which do not absorb water. The fabric is preferably made from nylon, Dacron, or other suitable synthetic fibers. It may, however, also be made of natural fibers suitably treated, as by resins, to render them non-absorbent of water. The fabric, which may be of the type disclosed in US. Patent No. 2,903,021 to Holden et al., has particular characteristics which enable elfective dewatering. The fabric is desirably made in a manner which maximizes permeability and which minimizes the void volume for any particular required strength.

In this connection, the void volume of the fabric is functionally related to the diameter of the yarn comprising the fabric and to the thread count. The thread count of the fabric is generally dictated by other desirable functional characteristics of the fabric, such as strength, abrasion resistance, and permeability. Accordingly, yarn diameter is an important factor in minimizing the void volume of the fabric, The fibers of the fabric may comprise multifilament or monofilament strands,rbut desirably should not comprise a substantial proportion of staple fibers since the latter provide a somewhat compressible fabric of which the permeability and void volume will vary with pressure.

The wet web is transferred to the permeable carrier and passed between one or more sets of pressure rolls, at least the one contacting the permeable carrier being of open construction. The particular construction of the open rolls is not of substantial importance in connection with the practice of the invention. However, in order to obtain best results with very fine fabrics, the openings in the roll surface should be relatively uniformly spaced and the solid area should be so proportioned that the required length of travel along the solid area be as short as possible consistent with roll strength so as to minimize the possibility of the water expressed from the web running back along the web to form a pond at the inlet side of the nip. H

Generally more than one pressure nip is required because initially the water content of the Web is so great that, if enough pressure is exerted to express substantially all of the water atone nip, it cannot flow away fast enough at'high speeds'to avoid crushing.

After passing through one or more pressure nips and after the desired solids level is reached in the paper web,

i.e., the consistency of the web is proper for the next succeding operation, the sheet is transferred to a subsequent press fabric or to the dryer rolls. After the sheet is transferred from the carrier, the carrier may be cleaned, as with water showers, and dried, as with suction boxes or air showers, after which it is returned to receive another portion of the wet web for dewatering.

The mechanism of dewatering the web in accordance with the present invention takes the wet web, the fiber mat of which mechanically holds water between its fibers, and presses it in contact with the permeable carrier which is substantially incompressible and whose permeability and void volume are practically constant. During the pressing, the paper fibers are compacted thereby causing any water contained between the fibers to be expressed from the passageways between the fibers. This water flows through the permeable carrier which, as has been pointed out, is sufliciently permeable that the water contained in the wet paper web and expressed by the pressure between the rolls can flow through the openings in the carrier in the time that the carrier and web pass through the nip under the effect of the pressure of the nip. The permeability of the carrier and its support on an open roll permits the water to flow through the web and into the carrierin a direction substantially perpendicular to the plane of the web and carrier, thus minimizing lateral how in the web and avoiding crushing. Thereafter, as the web and carrier pass the nip and the pressure is released, the paper web reabsorbs at least a portion of the moisture left in the carrier. It is for this reason that the void volume is made low. It should be low enough that the carrier cannot contain more water per unit area than the amount which when returned to the web leaves a proper consistency in the web as it leaves the carrier. Under these conditions the excess water expressed into the carrier is forced out through the openings in the open roll, thereby being permanently removed.

It has been found that, when a system in accordance with the invention is employed, the criticality of pressure on a web sheet is minimized and that pressures may be increased substantially without danger of crush. Further, fines retention in the sheet is maximized. As a result of the practice of the invention the sheet is more effectively dewatered in the press section making possible higher machine speeds or reduction in heat required in the dryers or both.

The following specific examples are illustrative of fabrics and operating conditions, as well as results which occur when practicing various of the features of the invention.

Example I A corrugating medium sheet having a basis weight of about 26 pounds per 1000 square feet was made upon a papermaking machine operating at a speed of approximately 605 feet per minute. The machine was first operated with a standard felt in the press section and then Was operated in accordance with this invention employing a carrier fabric. The carrier fabric which was employed was woven from 10 strands of 14 denier per strand nylon in the machine direction and 10 strands of 14 denier per strand nylon in the cross direction. The thread count of the fabric Was per inch in the machine direction and 60 per inch in the cross direction. The thickness of the woven fabric was approximately 0.0135 inch, the void volume of the fabric was determined to be 127 cubic inches per hundred square feet, and the permeability of the fabric was determined to be 10 gallons per minute of water per square inch at 4 pounds of pressure per square inch. '.The press rolls employed in the machine were 26%- inches in diameter and the roll in contact with the fabric was of open construction. More particularly, the roll included alternate bands and open areas of uniform spacing such that the open space area was approximately 40 percent of the total.

The results of comparative sheet consistencies when the sheet was supported on the conventional felt and upon the carrier fabric described above are set forth in the following table:

*Sheet solids, percent by weight.

As will be seen from the foregoing table, improved dewatering :to the extent of an increase of 4 percent solids was obtained when employing the principles of the present invention as compared to the use of a standard felt.

Example II A pulp sheet having a basis weight of about 175 pounds per 1,000 square feet was made upon a machine operating at a speed of approximately 323 feet per minute. The machine was first operated with a standard felt in the press section and then was operated in-accordance with this invention employing a carrier fabric. The carrier fabric which was employed was woven from strands of 14 denier per strand of nylon in the machine direction and 10 strands of 14 denier per strand of Dacron in the cross direction. The thread count of the fabric was 110 per inch in the machine direction and 60 per inch in the cross direction. The thickness of'the woven fabric was approximately 0.0135 inch, the void volume of the fabric was determined to be 116 cubic inches per hundred square feet, and the permeability of the fabric was determined to be 10 gallons per minute of water per square inch at 4 pounds of pressure per square inch.

The press rolls employed in the machine were 26 /2 inches in diameter and the roll in contact with the fabric was of open construction. More particularly, the roll included alternate bands and open areas of uniform spacing such that the open space area was approximately. 40 percent of the total.

The results of comparative sheet consistencies when the sheet was supported on the conventional felt and upon the carrier fabric described above are set forth in the following table:

First press Secondpress Machine Couch Sheet speed, eonsupport feet per sist- Pressure, 0011- Pressure, Conminute eney* lbs/tin. sistlbs./lin. sistinch eney* inch eucy* Felt 323 20. 4 150 27. 0 170 29 Fabric as i above (lescribed. 335 20. 3 150 31. 0 170 32. 9

A corrugatins medium sheet having a basis Weight of about 86 pounds per 1,600 square feet was made upon a machine operating at a speed of approximately 580 feet er minute. The machine was first operated with a standard felt in the press section and then was operated in accordance with this invention employing a carrier fabric. The carrier fabric which was employed was woven from strands of 4.4 denier per strand Dacron in the machine direction and 60 strands of 0.009 mil monofilament nylon in the cross direction. The thread count of the fabric was 41 per inch in the machine direction and 60 per inch in the cross direction. The thickness of the woven fabric was approximately 0.028 inch, the void volume of the fabric was determined to be 248 cubic inches per hundred square feet, and the permeability of the fabric was determined to be 22 gallons per minute of water per square inchat 4 pounds pressure per square inch.

The press rolls employed. in the machine were 26 /2 inches in diameter and the. roll in contact with the fabric was of open construction. More particularly, the roll included alternate bands and open areas of uniform spacing. such that the open space area was approximately 40 percent of the total. The results of comparative sheet consistencies when the sheet was supported on the conventional. felt and upon the carrier fabric described above are set forth in the following table:

*Sheet solids, percent by weight.

As will be seen from the foregoing table, improved dewatering to. the extent of an increase of almost 4 percent solids was obtained when employing the principles of the present invention as compared to the use of a standard felt, andresulted in a reduction in heat required in the. dryer section.

As is shown from the foregoing examples, there has been provided improved means for dewatering the web on a papermaking machine whichresults in economies of operation and/or higher machine speeds.

Various of the features of the present invention are set forth in the following claims.

What is claimed is:

l. A method of removing water from a wet web in the press section of a papermaking machine to reduce the amount of water in the web to a degree such that the consistency of the web is proper for the next succeeding operation comprising the steps of disposing the wet web directly on an endless permeable, porous carrier that has a void volume no greater than the amount of water which when returned to the web after pressing leaves the web with the proper consistency, the permeability and void volume of the "carrier being substantially independent of the pressure exerted thereon; passing the carrier and the web thereon between a pair of press rolls, at least one of which is of] open construction, the carrier being between the web and an open press roll; pressing the pair of press rolls together with pressure sufficient to move water through the web substantially perpendicularly of its surface into the pores of the carrier and through the carrier into openings in the open press roll; and continuing to support the web on the carrier after pressing.

2. A method of removing. Water from a wet web in the press section of a papermaking machine to reduce the amount of water in the web to a degree such that the consistency of the web is proper for the next succeeding operation comprising the steps of disposing the wet web directly on an endless porous carrier that is. permeable both perpendicularly to its surface and laterally and has a void volume no greater than the amount of water which when returned to the web after pressing letves the web with the proper consistency, the permeability and void volume of the carrier being substantially independent of the pressure exerted thereon; passing the carrier and the web thereon between a pair of press rolls, at least one of which is of open construction, the carrier being between the web and an open press roll; pressing the pair of press rolls together with pressure sufficient to move water through the web substantially perpendicularly of its surface into the pores of the carrier and laterally and perpendicularly through the carrier into openings in the open press roll; and continuing to support the web on the carrier after pressing.

3. A method of removing water from a wet web in the press section of a papermaking machine to reduce the amount of water in the web to a degree such that the consistency of the web is proper for the next succeeding operation comprising the steps of disposing the wet web directly on an endless porous carrier that is permeable both perpendicularly to its surface and laterally and has a void volume no greater than the amount of water which when returned to the web after pressing leaves the web with the proper consistency, the permeability and void volume of the carrier being substantially independent of the pressure exerted thereon; passing the carrier and the web thereon between a pair of pres rolls, at least one of which is of open construction, the carrier being between the web and an open press roll; pressing the pair of press rolls together with pressure sufiicient to move water through the web substantially perpendicularly of its surface into the pores of the carrier and laterally and perpendicularly through the carrier into openings in the open press roll, said pressure being insufiicient to produce any substantial lateral movement of water in the web as it passes through the nip between the pair of press rolls; and continuing to support the web on the carrier after pressing.

4. A method of removing water from a wet web in the press section of a papermaking machine to reduce the amount of water in the web to a degree such that the consistency of the web is proper for the next succeeding operation comprising the steps of disposing the wet web directly on an endless porous carrier that is permeable both perpendicularly to its surface and laterally and has a void volume no greater than the amount of water which when returned to the web after pressing leaves the web with the proper consistency, the permeability and void volume of the carrier being substantially independent of the pressure exerted thereon; passing the carrier and the web thereon between a pair of press rolls, at least one of which is of open construction, the carrier being between the web and an open press roll; pressing the pair of press rolls together with pressure suificient to move water through the web substantially perpendicularly of its surface into the pores of the carrier and laterally and perpendicularly through the carrier into the openings in the open press roll, the rate at which the carrier and the web are passed between the pair of press rolls being insutficient at the pressures used to produce any substantial lateral movement of water in the web as it passes through the nip between the pair of press rolls; and continuing to support the web on the carrier after pressing.

5. Apparatus for removing water from a wet web in the press section of a papermaking machine to reduce the amount of water in the web to a degree such that the consistency of the web is proper for the next succeeding operation comprising a pair of press rolls, at least one of which is of open construction, an endless permeable porous carrier that has a void volume no greater than the amount of water which when returned to the web after pressing leaves the web with the proper consistency, the permeability and void volume of the carrier being substantially independent of the pressure exerted thereon, means for supporting said carrier for endless movement, means for disposing the wet web directly on said carrier, means for passing said'carrier with the web thereupon between said pair of press rolls with said carrier adjacent an open press roll, and means for pressing said pair of press rolls together with pressure sur'licient to move water through the web substantially normally of its surface into the pores of the carrier and through the carrier into openings in the open press roll, said carrier supporting means disposing the carrier so as to support the web after leaving the press rolls.

6. Apparatus for removing water from a wet web in the press section of a papermaking machine to reduce the amount of water in the web to a degree such that the consistency of the web is proper for the next succeeding operation comprising a pair of press rolls, at least one of which is of open construction, an endless porous carrier that is permeable both perpendicularly to its surface and laterally and has a void volume no greater than the amount of water which when returned to the Web after pressing leaves the web with the proper consistency, the permeability and void volume of the carrier being substantially independent of the pressure exerted thereon, means for supporting said carrier for endless movement, means for disposing the wet web directly on said carrier, means for passing said carrier with the web thereupon between said pair of press rolls with said carrier adjacent an open press roll, and means for pressing said pair of press rolls together with pressure sufiicient to move Water through the web substantially normally of its surface into the pores of the carrier and laterally and perpendicularly through the carrier into openings in the open press roll, said carrier supporting means disposing the carrier so as to support the Web after leaving the press rolls.

7. Apparatus for removing water from a wet web in the press section of a papermaking machine to reduce the amount of water in the web to a degree such that the consistency of the web is proper for .the next succeeding operation comprising a pair of press rolls, at least one of which is of open construction, an endless, porous carrier that is permeable both perpendicularly to its surface and laterally and has a void volume no greater than the amount of water which when returned to the web after pressing leaves the Web with the proper consistency, the permeability and void volume of the carrier being substantially independent of the pressure exerted thereon, means for supporting said carrier for endless movement, means for disposing the wet web directly on said carrier, means for passing said carrier with the web thereupon between said pair of press rolls with said carrier adjacent an open press roll, and means for pressing said pair of press rolls together with pressure sutrlcient to move water through the web substantially normally of its surface into the pores of the carrier and laterally and perpendicularly through the carrier into openings in the open press roll, the permeability of said carrier being sufficient to permit substantially all of the water expressed from the web to flow into said carrier during the time it passes through the nip between said pair of press rolls, said carrier supporting means disposing the carrier so as to support the web after leaving the press rolls.

8. Apparatus for removing water from a wet web in the press section of a paper-making machine to reduce the amount of water in the web to a degree such that the consistency of the web is proper for the next succeeding operation comprising a pair of press rolls, at least one of which is of open construction, an endless substantially incompressible, porous carrier Woven of synthetic fibers that do not absorb water, said carrier being permeable both perpendicularly to its surface and laterally and having a void volume no greater than the amount of water which when returned to the web after pressing leaves the web with the proper consistency, the permeability -and void volume of the carrier being substantially independent of the pressure exerted thereon, means for supporting said carrier for endless movement, means for disposing the wet web directly on said carrier,

means for passing said carrier with the web thereupon between said pair of press rolls with said carrier adjacent an open press roll, and means for pressing said pair of press rolls together with pressure suflieient to move water through the web substantially normally of its surface into the pores of the carrier and laterally and perpendicularly through the carrier into openings in the open press roll, said carrier supporting means disposing the carrier so as to support the web after leaving the press rolls.

9. Apparatus for removing water from a wet web in the press section of a papermaking machine to reduce the amount of water in the web to a degree such that the consistency of the web is proper for the next succeeding operation comprising a pair of press rolls, at least one of which is of open construction, an endless substantially incompressible, porous carrier woven of fibers that do not absorb water, said carrier being permeable both perpendicularly to its surface and laterally and having a void volume no greater than the amount of water which when returned to the web after pressing leaves the web with the proper consistency, the permeability and void volume of the carrier being substantially independent of the pressure exerted thereon, means for supporting said carrier for endless movement, means for disposing the wet web directly on said carrier, means for passing said carrier With the web thereupon between said pair of press rolls with said carrier adjacent an open press roll, and means for pressing said pair of press rolls together with pressure suflieient to move water through the web substantially normally of its surface into the pores of the carrier and laterally and perpendicularly through the carrier into openings in the open press roll, said carrier supporting means disposing the carrier so as to supports the web after leaving the press rolls.

References Cited in the file of this patent UNITED STATES PATENTS 1,600,509 Millspaugh Sept. 21, 1926 1,978,982 Barstow Oct. 30, 1934 1,991,346 Ellis Feb. 12, 1935 3,093,535 Brauns et a1. June 11, 1963 FOREIGN PATENTS 1,198,432 France June 15, 1959 624,831 Canada Aug. 1, 1961 

1. A METHOD OF REMOVING WATER FROM A WET WEB IN THE PRESS SECTION OF A PAPERMAKING MACHINE TO REDUCE THE AMOUNT OF WATER IN THE WEB TO A DEGREE SUCH THAT THE CONSISTENCY OF THE WEB IS PROPER FOR THE NEXT SUCCEEDING OPERATION COMPRISING THE STEPS OF DISPOSING THE WET WEB DIRECTLY ON AN ENDLESS PERMEABLE, POROUS CARRIER THAT HAS A VOID VOLUME NO GREATER THAN THE AMOUNT OF WATER WHICH WHEN RETURNED TO THE WEB AFTER PRESING LEAVES THE WEB WITH THE PROPER CONSISTENCY, THE PERMEABILITY AND VOID VOLUME OF THE CARRIER BEING SUBSTANTIALLY INDEPENDENT OF THE PRESSURE EXERTED THEREON; PASSING THE CARRIER AND THE WEB THEREON BETWEEN A PAIR OF PRESS ROLLS, AT LEAST ONE OF WHICH IS OF OPEN CONSTRUCTION, THE CARRIER BEING BETWEEN THE WEB AND AN OPEN PRESS ROLL; PRESSING THE PAIR OF PRESS ROLLS TOGETHER WITH PRESSURE SUFFICIENT TO MOVE WATER THROUGH THE WEB SUBSTANTIALLY PERPENDICULARLY OF ITS SURFACE INTO THE PORES OF THE CARRIER AND THROUGH THE CARRIER INTO OPENINGS IN THE OPEN PRESS ROLL; AND CONTINUING TO SUPPORT THE WEB ON THE CARRIER AFTER PRESSING. 